The present invention relates to shiplifts, and in particular, to a method of operating a shiplift.
A shiplift generally includes two rows of hoists connected on opposite sides of a lifting platform. The hoists can be of many types, including electrically or hydraulically driven winches or hydraulic rams, and can be connected to the platform in alternative manners, including by wire rope or chain. The number and size of hoists employed can be varied as desired depending on the load to be lifted. A typical shiplift will utilize between 4 and 110 hoists.
The platform of a shiplift can be rigid or, as supplied by the assignee of the present application, can be articulated such that portions of the platform can be moved vertically relative to other portions of the platform. In a platform of the type typically used by the assignee of the present invention, the platform includes a plurality of main transverse beams (“MTBs”) that are able to articulate with respect to one another within a specified range of movement. Each MTB is supported between two hoists connected at opposite ends of the MTB. The MTBs are connected together in a known manner to form the platform while still allowing relative movement between respective MTBs. In some circumstances, the platform can be constructed of two or more sections that can be operated together for lifting larger ships/vessels, or can be operated independently of one another for independently lifting two or more smaller ships/vessels.
An example of a prior art shiplift to with which the present invention can be used is described in U.S. Pat. No. RE37,061, “Method of Distributing Loads Generated Between A Ship And A Supporting Dry Dock”, assigned to the assignee of the present invention and shown herein in FIGS. 1-4. Referring to FIG. 1, a platform 13 of the kind described in U.S. Pat. No. 4,087,979 supports a ship 9 for vertical movement with respect to a quay 10 (FIG. 2). Referring now to FIG. 2, the platform 13 includes a plurality of MTBs 20, the ends of which lie within cutouts 17 in the opposing faces of the quays 10 (FIG. 1) and 12 (FIG. 4). The ends of the beams 20 carries sheaves 18.
A plurality of opposed pairs of hoists are used, here in the form of hoist winches 19. See FIG. 4. Each hoist winch 19 is fixed to its respective quay and supports a further sheave 21 in approximately vertical alignment with the sheaves 18, and further includes a winch drum 29. See FIGS. 2 and 3. A wire rope 27 is fixed by one end to a load cell 25 which also doubles as a clevis pin and is fixed to the end of the structure of the hoist winch 19. The rope 27 is wrapped around the sheaves 18 and 21, the remaining end exiting sheaves 18 and being connected to the winch drum 29. Each winch drum 29 is driven by an ac synchronous motor 33 via a step down gear arrangement 35 and a toothed wheel 37 on the end of the drum 29. A limit switch 41 is fastened to the structure of the hoist winch 19 and a contact pad 43 is carried by the beam 20. The limit switch is pre-set and when the platform 13 rises to its desired height during operation, the pad 43 contacts the limit switch 41 which then is actuated to effect halting of the platform 20. Devices (not shown) within the system are utilized to determine the maximum desired lowered positions of the platform 13.
During operation of the hoist winches 19 to raise or lower the platform 13 and its associated ship 9, a conditioning circuit 28 receives electrical signals from the load cell 25 associated with that winch 19. See FIG. 4. The output from each circuit 28 is sent to computer/CPU 47. The computer 47 can process the data received and send control signals to the shiplift control panel, stopping or allowing operation of the hoist winches 19, and can send further signals to a visual display unit 49 so as to display information concerning the operating performance of the hoist winches 19, e.g. the loads being sensed, and also the current being drawn by the winch motor 33, the weight of the vessel being lifted/lowered and other characteristics of the system.
FIG. 5 shows a display in both histogram and numerical form of the distribution of a particular ship's weight over the hoist winches 19. Opposed winch stations 1A and 1B are each experiencing a load of 73.8 tons. Stations 4A and 4B are each experiencing a load of 256 tons and stations 6A and 6B are each experiencing a load of 72 tons. The weights indicated from zero upwards relate to the ship. The projections of the histogram below the zero line are identical in extent, and correspond to the constant weight of the platform.
The foregoing description discloses the use of a load cell 25 in the form of a clevis pin. However, other forms of load cell may be used, and positioned anywhere in the load path of the loads which the hoist winches 19 experience during operation. Thus, by way of example, load cells can be positioned on the support structure 51 of the hoist winch sheaves 21, or at 53 between the hoist winches 19 and the quays 10 and 12, or at the clevis pin supports. i.e., through use of a normal clevis pin 25 supported on a load cell of appropriately adapted shape.
A known shiplift control system supplied by the assignee of the present application, marketed under the name ATLAS™, provides shiplift operating information to the shiplift operator. For instance, it includes a calculated load distribution screen that indicates the probable distributed load of a vessel calculated from data input by the operator. If any distributed load is above the maximum designed distributed load, the monitor will display a warning that the vessel may overload the shiplift and should not be docked. If a warning is indicated, the distribution of the vessel load on the blocks may be changed by moving the center of gravity closer to the centerline of the loaded blocking. The following docking parameters are entered by the operator:
W=The ship load.
LK=The length of blocks to be bearing the keel.
A=The distance of the first block to the shore bulkhead in meters (feet).
LCG=The distance from the center of gravity of the ship to the shore bulkhead.
The setting limits will be shown in a window of the display, together with an input setting box for the value input. The display will show the calculated load distribution for the vessel to be docked.
The ATLAS™ system also includes a center of gravity mode which provides information on the vessel's longitudinal and transversal center of gravity on the platform and the shipload on each main transverse beam.
This information can be used by the operator to identify any docking abnormalities such as incorrect vessel positioning.
U.S. Pats. RE36,971, “Method Of Determining And Analyzing A Ship's Weight” and RE37,061, “Method of Distributing Loads Generated Between A Ship And A Supporting Dry Dock”, both describe methods of operating a shiplift.
U.S. Pat. Nos. 3,073,125, 4,087,979, RE36,971 and RE37,061, all related to shiplifts and assigned to the assignee of the present invention or corporate predecessors, are incorporated by reference herein.